1. Field of the Invention
This invention relates to a method of producing galvanneal .delta..sub.1 Zn-Fe alloy coatings on a steel substrate, and the product thereby formed.
2. Discussion of Background
As described in U.S. Pat. No. 4,640,872 to Irie et al., among surface treated streel strips, zinc coated steel has found the widest variety of applications, for example, in automobiles, electric appliances, building material and the like because of its improved sacrificial corrosion prevention effect. Recently, the need for rust prevention has been increased in some applications and it has been desired to enhance the rust prevention of zinc coated steel. There has been the need for imparting heavy duty rust prevention to zinc coated steel because the rust prevention that current zinc-coated steel possesses is still insufficient in certain applications. More illustratively, zinc coated steel strips have poor phosphatability, paintability, and wet adhesion of paint coating, and deteriorate in corrosion resistance during service at joints such as hemmed joints as often formed in automobile doors whether or not they are coated with paint. A closer attention has been paid to these drawbacks and there is the strong desire to overcome them. Particularly, surface treated steel strips for use in automobiles are required to have improved corrosion resistance with or without paint coating, particularly improved perforation corrosion resistance at joints as well as good weldability, workability, phosphatability and paintability.
Among prior art conventional surface treated steel strips, there are known galvannealed steel strips which satisfy the above requirements to some extent as they possess exceptionally high corrosion resistance after paint coatings. The conventional galvannealed steel is prepared by subjecting steel to zinc hot-dipping followed by a heat treatment to form a Zn-Fe alloy coating having a major proportion of .delta..sub.1 phase. Zinc electroplated steel has also been used to form a galvanneal product by an isothermal heat treatment to produce similar results.
Thus, the .delta..sub.1 phase structure is usually produced by the hot-dip and annealing process. This material, called .delta..sub.1 galvanneal, is known to have the best ductility and corrosion resistance of all the intermetallic structures produced in the Fe-Zn system and consists of 10% .GAMMA. phase and 90% .delta. phase. In the conventional hot-dip galvanneal process, the aim is to alter favorably the ratio of the phase layers present for better coating properties.
Two methods typically used to produce .delta..sub.1 galvanneal are:
(1) Heat the Zn coated steel strip immediately after it leaves the galvanizing bath and before the zinc coating has solidified. This structure typically contains 10% .GAMMA. phase and 90% .tau..sub.1 phase.
(2) Heat galvanized steel below the melting point of zinc up to 350.degree.-380.degree. C. for 2-3 hours.
Most galvannealed steel sheets are produced in continuous galvanizing lines using the first method. The galvanneal coatings exhibit good paint adhesion because its surface is relatively uniform and smooth with a finish on a microscale which gives relatively good mechanical keying for painting. The coating has relatively good corrosion resistance after painting and is easier to weld than galvanized coatings. However, the conventional techniques for producing .delta..sub.1 galvanneal have been unsuccessful in providing steel strips with such a high degree of strength and workability as is currently required for automobile use. Further, when thinly coated, the conventional galvannealed steel strips do not possess satisfactory local corrosion resistance or perforation corrosion resistance during service at joints like hemmed joints.
In order to eliminate the above-mentioned shortcomings of galvannealed steel while taking advantage of its excellent corrosion resistance with or without paint coating, Zn-Fe alloy electroplating has recently been used as an improvement over the galvannealing as disclosed in Japanese Patent Application Kokai Nos. SHO 54-107838, 57-60087 and 57-200589, and Japanese Patent Publication No. SHO 57-61831, for example. The Zn-Fe alloy electroplating is substantially equivalent to galvannealing in regard to corrosion resistance with or without paint coating, paint adhesion, phosphatability and weldability where the content of iron is in the range of 5% to 30% by weight.
Unfortunately the prior art techniques for producing galvannealed steel sheets have produced products which are not entirely satisfactory. It is believed by the present inventors that one important shortcoming of the prior techniques is that these techniques produce, in addition to .delta..sub.1 phase Zn-Fe structure, significant amounts of other Zn-Fe phases, particularly, .GAMMA. phase, which diminish the corrosion resistance, paint adhesion and weldability of the finished product. The prior art techniques for producing a galvannealed steel sheet from Zn-Fe alloy electrodeposited coatings are only concerned with an isothermal galvannealing process and do not involve an in-line processing technique whereby the electroplated steel is dynamically heated to a predetermined temperature and then cooled to room temperature.
Other prior art references of interest to the background of the present invention are U.S. Pat. No. 4,252,866 and Japanese patent publications 55-37590, 56-13490, 57-19393, 57-19331, 57-89494, 57-164998, 57-200589, 58-117866, 59-23894, 59-200791 and 59-229493.